This invention relates to coated metal particles having a coat forming substance applied to the surfaces of metal particles. The invention also relates to a process for producing metal-base sinters by sintering those coated metal particles or mixtures containing those particles, as well as metal-base sinters produced by the process.
To enhance the performance of metal-base sinters, providing them with an enhanced composite formulation and structure is insufficient and it is also necessary to refine and homogenize their structure. Energetic R&D efforts have been made to meet this need and as for metals that are fairly stable in atmosphere, particles with diameters no more than 10 .mu.m are used extensively. As for active metals and those metals under active study in recent years which do not exhibit satisfactory chemical stability when formulated into intermetallic compounds or processed into a powder of fine particles, the commonly used particles exceed 10 .mu.m in diameter.
In either case, the objective of enhancing the performance of metal-base sinters makes it strongly desirable that the particles be refined or prevented as much as possible from increasing in diameter, that the particles be given a composite structure by dispersive addition of dissimilar substances, and that the composite microstructure be homogenized.
To manufacture such metal-base sinters, the powder metallurgical technology is indispensable. In powder metallurgy, preparing an optimal feed powder for the manufacture of the desired metal-base sinter is important and it is essential that substances that are similar to the substrate and which are capable of enhancing the performance of metal-base sinters and/or substances dissimilar from the substrate and having the same capability, say, those substances which will not react chemically with the metal particles, or the reaction products that form as a result of reaction with the metal particles should be present on the surfaces of those particles so that they are distributed uniformly throughout the interior of the sinter.
A method that permits the addition of such substances at the stage of the preparation of the feed powder is mechanical alloying. This method involves mixing the metal with ceramics, alloys, intermetallic compounds, etc. and kneading the mixture with an attritor, or a powerful ball mill loaded with steel balls. Through the steps of crushing the metal particles during kneading and subsequent adhesion of another kind of particles to the crushed particles, the other particles can be added in such a way that they are dispersed in the metal particles. Mechanical alloying has the added advantage of providing a great latitude in the selection of such parameters as the size and kind of the powder particles and, hence, this technique has heretofore been used by many manufactures.
However, the occurrence of unavoidable incorporation of impurities is more frequent in mechanical alloying than in the usual powder mixing approach; what is more, there is theoretically a limit on the extent to which a uniform structure can be realized. Even if the particles of feed substances such as similar or dissimilar metals, ceramics, intermetallic compounds, etc. which will eventually be incorporated into the desired metal-base material are fine, it is very difficult to insure ideal mixing or uniform dispersion in such a way that the particles of the feed substances will cover all of the metal particles present. Even if such uniform dispersion is realized, the extent of "uniformity" that can be attained on a microscopic level is limited since the particles of the feed substances are mixed on a particle basis. Particularly in the case where those particles are used in relatively small amounts, an uneven distribution will occur inevitably.
In many actual cases, the metal particles in powder form and/or the particles of said feed substances will agglomerate to form lumps in the sinter of the metal-base material or they occur unevenly in the sinter, eventually leading to marked deterioration in the performance of the metal-base sinter.
Therefore, in order to realize homogenization, it is necessary that the feed substances which will eventually be incorporated into the intended metal-base sinter should be distributed positively to all individual metal particles. In addition, the coating on the surfaces of the metal particles must be highly controlled and uniform, namely, coatings of uniform morphology must be provided over the surfaces of individual metal particles and it is further required that such uniform coatings cover every one of the metal particles present. What is more, if the particles have an increased diameter, the highly controlled uniform coating is required to be completely uniform in that they are practically free from uncoated portions.
Thus, it is strongly desired to manufacture coated metal particles that are provided with uniform coats as controlled in accordance with their diameter and to manufacture high-performance metal-base sinters using such coated metal particles.
While feed substances for coatings can be applied by various techniques such as vapor-phase processes and wet plating methods, the vapor-phase approach has major features that are unattainable by other coating techniques, such as (1) easy control of the atmosphere; (2) the selection of coat forming substances which are provided by said feed substances which are eventually incorporated into the intended metal-base sinter is basically unlimited and various kinds of substances including elemental metallic substances (e.g. active metals), nitrides, carbides, borides and oxides can be applied; (3) the desired coat forming substance can be applied without letting impurities be incorporated; and (4) the coating weight of the coat forming substance can be controlled freely.
However, for the reasons to be set forth below, none of the various coating apparatus and methods heretofore proposed as known techniques have been capable of forming uniform coatings controlled in accordance with the diameter of particles.
First, in the case of a core particle powder composed of fine metal particles with diameters of no more than 10 .mu.m, the individual metal particles are cohesive enough to have a great tendency to agglomerate together, whereby almost all single particles form agglomerates. Since these agglomerates cannot be disintegrated unless they are subjected to a special action greater than their cohesive force, they cannot be simply coated as such to insure that the surfaces of the individual particles are covered with the coatings of said coat forming substances, eventually yielding coated agglomerates in which the surfaces of the agglomerates are covered with the coatings of the coat forming substances. This has caused a problem with the individual agglomerate-forming particles in that the surfaces of the particles located on the surfaces of the agglomerates have large coating weights but suffer from uneven coating whereas the particles located within the agglomerates are not covered at all.
With a core particle powder composed of metal particles exceeding 10 .mu.m in diameter, the cohesive force is not as strong as what develops in fine grains having diameters of no more than 10 .mu.m and, yet it has been impossible to insure that the core metal particles will become discrete to occur on a single-particle basis. Hence, those portions of agglomerates which are blocked by other metal particles remain uncoated on the surfaces. As already mentioned, uniform coatings are needed that are controlled in accordance with the particle diameter and, yet even the small cohesive force discussed above has been so much influential as to cause a very serious problem in actual cases.
With a view to solving these problems, attempts have already been made to coat the metal particles in a dispersed state in order to assure the coating of the surfaces of the individual particles in the powder of core particles.
For instance, Unexamined Published Japanese Patent Application (kokai) Sho 58-31076 teaches an apparatus and method, according to which a vessel placed in PVD equipment is charged with the particles in a powder of core particles and vibrated by an electromagnetic means so that the core particles in the vessel are rolled as they are coated by a PVD process. Unexamined Published Japanese Patent Application (kokai) Sho 61-30663 teaches an apparatus, according to which a vessel placed in PVD equipment is charged with the particles in a powder of core particles and vibrated by a mechanical means so that the core particles in the vessel are rolled as they are coated by a PVD process. However, in the actual practice with those apparatus or methods in which the vessel is vibrated so that the particles in the powder of core particles which are metal particles are rolled as they are provided with coatings, the necessary action for disintegrating the agglomerates of metal particles with an average diameter not greater than 10 .mu.m by applying a force exceeding their cohesive force cannot be produced and, hence, the agglomerates cannot be disintegrated; to the contrary, a granulating action develops to form agglomerates that are greater in number or size than before the powder of core particles is supplied into the vessel. On the other hand, metal particles having an average diameter in excess of 10 .mu.m are simply subjected to a sliding action as they form many layers in superposition and it has been impossible to achieve the desired coating of single separate particles.
Unexamined Published Japanese Patent Application (kokai) Hei 3-153864 teaches an apparatus and method, according to which a rotating vessel having barriers and/or ridges and grooves in the inner surface is charged with core particles and rotated as the surfaces of the particles are coated by an evaporation method. The problem with this apparatus and method is that the necessary action for disintegrating the agglomerates of metal particles with an average diameter not greater than 10 .mu.m by applying a force exceeding their cohesive force cannot be produced and, hence, the agglomerates cannot be disintegrated and, what is more, an increased number or size of agglomerates will simply form. On the other hand, metal particles having an average diameter in excess of 10 .mu.m are simply subjected to a gentle stirring action as many of them contact one another forming many layers in superposition and it has been impossible to achieve the desired coating of single separate particles.
Unexamined Published Japanese Patent Application (kokai) Sho 58-141375 teaches an apparatus in which the particles of a powder in a reactive gas atmosphere are suspended by the flow of the reactive gas under gravity and in which the surfaces of the particles are coated with the precipitating substance that forms by the chemical reaction involving the reactive gas. Unexamined Published Japanese Patent Application (kokai) Hei 2-43377 teaches a method in which particles placed under vacuum are fluidized as they are subjected to coating by a thermochemical reaction treatment. Unexamined Published Japanese Patent Application (kokai) Sho 64-80437 teaches a method in which the agglomerates of core particles in powder are disintegrated by a sound wave that is a composite of low and high frequency waves, so that the agglomerates are fluidized to improve the coating efficiency. However, these techniques which utilize the fluidized bed of the particles in a powder of core particles which is formed by a gas flow or vibrations have had the problem that with metal particles having an average diameter of no more than 10 .mu.m, it is practically impossible to fluidize the separate individual metal particles, thus failing to disintegrate the agglomerates of these metal particles. On the other hand, with metal particles exceeding 10 .mu.m in average diameter, it is practically impossible to insure that all of these metal particles are similarly and independently fluidized and suspended as single separate entities and one has been incapable of eliminating uneven coating of the particles which is due to the hiding of one metal particle by another.
Unexamined Published Japanese Patent Application (kokai) Sho 54-153789 teaches an apparatus in which a powder material is dropped within a vacuum vessel, where the metal vapor is generated to form a metal coating on the particles. Unexamined Published Japanese Patent Application (kokai) Sho 60-47004 teaches a method in which a monomer gas and the particles of a powder are introduced into a high-frequency plasma region in a vacuum vessel, where a coating film of an organic substance is formed by plasma-assisted polymerization. If metal particles with an average diameter of no more than 10 .mu.m are simply introduced as in the techniques described above, agglomerates of the metal particles cannot be disintegrated. On the other hand, metal particles exceeding 10 .mu.m in average diameter will simply drop while forming agglomerates which are not single separate particles and various problems occur, such as uneven coating due to the hiding of one particle by another, the total failure of the particles within an agglomerate to be coated, and differences in the coating weights of individual particles.
Unexamined Published Japanese Patent Application (kokai) Sho 64-80437 teaches a method in which the agglomerates of core particles in powder are disintegrated by a sound wave consisting of low and high frequency waves so that they are fluidized to improve the coating efficiency. However, this method which imparts vibrations to the fluidized bed has had the problem that with metal particles having an average diameter of no more than 10 .mu.m, it is practically impossible to fluidize the separate individual particles, thus failing to disintegrate the agglomerates of these particles. On the other hand, with metal particles exceeding 10 .mu.m in average diameter, it is practically impossible to insure that all of these particles are similarly and independently fluidized and suspended as single separate entities and one has been incapable of eliminating uneven coating of the particles which is due to the hiding of one particle by another.
Unexamined Published Japanese Patent Application (kokai) Sho 62-250172 teaches an apparatus and method, according to which a powder that has been preliminarily treated by jet milling is allowed to stay within a chamber for heat treatment under vacuum, where it is subjected to a heat treatment and thence dropped under gravity through a powder feeder into a cylinderal sputtering chamber equipped with a vertical target, whereby the particles in powder are provided with a coating. Unexamined Published Japanese Patent Application (kokai) Hei 2-153068 teaches an apparatus and method, according to which a powder that has been preliminarily treated by jet milling is allowed to stay within a chamber for heat treatment under vacuum, where it is subjected to a heat treatment and thence introduced through a powder feeder into a rotary vessel accommodating a sputter source within a sputtering chamber in the form of a powder (not as single particles), with sputtering being effected as the vessel is rotated. These techniques involve a heating step which is performed before coating so that the jet-milled powder of core particles of metal particels is allowed to stay for heat treatment and because of this staying of the powder in the heating step, the metal particles of any diameter will form agglomerates again which are not single particles and, eventually, such agglomerates will not revert to single particles in the coating step.
Thus, none of the so far proposed techniques have successfully solved the problems associated with the apparatus or method for providing coatings on the core particles in powder which are metal particles. In actual cases, metal particles having an average diameter of no more than 10 .mu.m form agglomerates which cannot be disintegrated and, hence, no methods or apparatus have been available for producing coated metal particles in which said metal particles, being dispersed as single particles, are covered on their surfaces with coat forming substances. Speaking of metal particles exceeding 10 .mu.m in average diameter, these particles are in actual cases subjected to a coating treatment in the form of agglomerates in which they remain in mutual contact and, hence, they have not ever been given highly controlled uniform coatings. In other words, no methods have been available for producing metal particles with highly controlled uniform coatings, nor have been apparatus for implementing such methods. Hence, irrespective of the diameter of starting metal particles, it has been impossible to prepare coated metal particles by providing each of those starting particles with a controlled uniform coat by vapor-phase coating techniques using bonding material forming substances and/or sintering aid forming substances and, it has accordingly been impossible to produce the aforementioned metal-base sinters of high performance.